There are many types of the UFS file system, those many are supported by the Linux Kernel but its very hard to automatically detect which type of UFS needs to be mounted, that is why you need to add the ufstype= argument to the mount command.

Possible common types are:

old old format of ufs

default value, supported as read-only

44bsd used in FreeBSD, NetBSD, OpenBSD

ufs2 used in FreeBSD 5.x

5xbsd synonym for ufs2

sun used in SunOS (Solaris)

sunx86 used in SunOS for Intel (Solarisx86)

hp used in HP-UX

nextstep used in NextStep

nextstep-cd used for NextStep CDROMs (block_size == 2048)

openstep used in OpenStep

Since you are comming from FreeNAS and that is based on FreeBSD I recommend you try ufs2 or 44bsd in the ufstype= parameter.